Adhesive materials that can support high capacity loads have traditionally been sought. Pressure sensitive adhesives (PSAs) are self-adhesives that form a bond when pressure is applied to marry the adhesive with the adherend. Typically, no solvent, water, or heat is necessary to activate or stimulate the adhesive to perform. Conventional PSAs have found use in pressure sensitive tapes, labels, note pads, automobile interior trims, and a wide variety of other products. There has been continued effort among the scientific community to create and produce synthetic materials and devices that mimic the performance of animals, such as geckos, in Nature.
PSAs typically rely on ubiquitous surface force interactions, such as van der Waals forces, to transfer stress across an interface. Pressure sensitive adhesives can adhere to a surface because the adhesive is soft enough to flow, or wet, the adherend. The adhesive needs to be hard enough to resist flow when stress is applied to the bond at the interface. PSAs exhibit viscoelastic (viscous and elastic) properties, both of which may be used for creating proper bonding.
Currently, PSAs are mainly fabricated from soft, viscoelastic polymer materials (e.g., coatings), which are used either independently or in conjunction with a backing material, such as a stiff film or cloth. To control the level of adhesion, PSAs alter their bulk properties by relying upon a complex formulation of both elastic and viscoelastic components to delicately balance the ability of a material to form an interface (or “wet a surface”) and the resistance to separation once the interface is formed. (Benedek, et al. Eds. 2009 Handbook of Pressure Sensitive Adhesives and Products Series, CRC Press: Boca Raton; Pocius, 2002, Adhesion and Adhesives Technology: An Introduction, Hanser Publ.: Munich; Crosby, et al. 1999 J. Poly. Sci. Part B: Polym. Phys. 37, 24, 3455-3472; Creton, 2003 “Materials Science of Adhesives: How to Bond Things Together.” MRS Bulletin 28, 6, 419-421; Creton, 2003 “Pressure-sensitive adhesives: An introductory course.” MRS Bulletin 28, 6, 434-439; Creton, et al. 2007 “Sticky Feet: From Animals to Materials”, MRS Bulletin 32, 6, all pages; Chan, et al. 2007 “Designing Model Systems for Enhanced Adhesion.” MRS Bulletin 32, 6, 496-503; Boesel, et al. 2010 Advanced Materials 22, 19, 2125-2137.)
Among the various aspects of PSA designs, three factors are typically relevant and emphasized: (1) Conventional PSAs are viscoelastic to allow the polymer coating to conform easily with rough a surface while dissipating mechanical energy (i.e., pressure) that is required for conformality; (2) A measure for strong PSA materials is tack energy, which is the total energy dissipated during the separation of a PSA/substrate interface; (3) High tack PSAs are typically not conducive to multiple loading applications due to the irreversible (i.e. inelastic) materials processes that are used to produce high levels of tack.
To develop adhesive material systems that overcome some of the disadvantages of conventional PSAs, much research has focused on the development of gecko-like adhesive systems. Some key attributes of both conventional PSAs and ideal gecko-like adhesives are provided in the following table:
ConventionalIdeal “Gecko-Like”PropertyPSAAdhesiveMax Shear StressHighHighMax Normal StressHighHighPeel ResistanceHighLow (after reachingcritical peel angle)Energy of SeparationHighLowReversibilityLimited to NoneHighTime/Temp DependenceHighUnknownImpact of FoulingHighLimited
Additionally, the adhesive surface of geckos, and similar examples in Nature, is commonly described as “dry”, i.e., that adhesion does not rely upon liquid interactions, liquid-to-solid transitions like an epoxy, nor does the adhesive surface feel “tacky” to the touch like a conventional viscoelastic adhesive. Although such attributes are known and displayed in Nature, the primary design factors or mechanisms that permit good control of properties is not known and remains the subject of current research projects worldwide. To our knowledge, the development of a synthetic analog to a gecko-like adhesive has not been demonstrated, in particular one that could be used on macroscopic length scales.
There is a significant and ongoing need for designs, systems, devices, materials and related fabrication methods for adhesive systems that can be used to easily attach and support high capacity loads, yet provide simple, non-damaging release and repeated use while being cost-effective to produce.